1. Field of the Invention
This invention relates to methods of treating oil and gas wells with carbon dioxide and, specifically, to an improved method for reducing the vaporization of carbon dioxide during the injection operation.
2. Description of the Prior Art
Various well stimulation techniques are known for increasing the production rate of oil and gas wells, including hydraulic fracturing and injection treatments with chemicals. In the fracturing treatment, a liquid slurry under high pressure is pumped into the well to cause cracks or fissures in the formation. Various proppants such as glass beads, sand particles, or the like, can be incorporated in the slurry to enter the cracks created by the high pressure fluid, to prop the cracks open for improved formation flow.
Carbon dioxide has been found to be advantageous for mixing with the well treating slurry. Among other advantages, carbon dioxide reduces swelling in clay sensitive formations, lowers the pH of the well treating fluid, and can be helpful in removing blocks from the formation. When carbon dioxide is used with a liquid treating solution, it is normally pumped in the liquid state. As the carbon dioxide exceeds its vaporizing temperature at the injection pressure, it begins changing into a vapor state, or gas. As the carbon dioxide undergoes the phase change to a gas, its volume increases dramatically. The carbon dioxide gas enters the perforations in the formation and forms a gas lift for lifting the treating fluids out of the well after the surface pressure is removed, thus facilitating cleanup operations.
U.S. Pat. No. 4,212,354 illustrates a problem associated with obtaining a sufficiently high flow rate of carbon dioxide to mix with the fracturing slurry being pumped into the well. The carbon dioxide is normally brought to the well site in a tank truck. The temperature is maintained at about -20.degree. F. to 0.degree. F., and the pressure is around 250 to 300 psig (pounds per square inch gage). Under these conditions, the carbon dioxide is liquid except for a small vapor blanket at the top of the tank.
In the past, the liquid carbon dioxide was drawn off to a booster pump, which would increase the liquid pressure about 50 to 125 psig above the tank pressure. The purpose of the booster pump was to reliquify partially vaporized carbon dioxide that vaporizes in the tank discharge line, due to the pressure drop and warming of the liquid. However, withdrawal of liquid from the tank causes the tank pressure to drop, and consequently more and more carbon dioxide vaporizes in the line. The vapor blanket in the tank increases in size, but the liquid in the tank does not vaporize fast enough to maintain the tank at a pressure sufficient to prevent "flashing" in the discharge line. The fast vaporization in the discharge line, or "flashing", absorbs the latent heat of vaporization and lowers the liquid temperature drastically. The temperature drop may go below the freezing point of carbon dioxide, freezing the liquid carbon dioxide and blocking the line. In addition, a large amount of vapor may case the booster pump to vapor lock. This would result in only small flow rates of carbon dioxide being supplied to the well head.
The phase changes discussed above can adversely affect the fracturing treatment, since the treatment cannot be temporarily stopped during the fracturing operation. Otherwise, the well will "sand up", due to the large amount of sand in the slurry on its way to the perforations. If the carbon dioxide booster pump vapor locks, or if the line freezes, the carbon dioxide must be deleted from the remainder of the well treatment.
In U.S. Pat. No. 4,212,354, a portion of the carbon dioxide in the discharge line is recirculated to the top of the tank after being heated in a vaporizer. By recirculating the vaporized carbon dioxide to the tank, the tank pressure is maintained substantially constant. While this scheme presents a solution to the problem, the booster pump is not eliminated, and some operators object to the presence of a heater at the well site.